The present invention relates to a fuel system provided in a carburetor for general purpose engines and, more particularly, to fuel systems designed so that the flow rate of a fuel supplied from a constant-fuel chamber to a nozzle orifice is mechanically adjusted in response to the open-close operation of a throttle valve, and the fuel is mixed with bleed air and discharged into an intake channel.
Because carburetors supplying fuel to general purpose engines are small, they mostly have simplified fuel systems. Well-known carburetors include fixed venturi carburetors using a single fuel system in which a nozzle orifice is opened in the narrowest portion of venturi tube, as described in Japanese Patent Application No. 46-10565, and variable venturi carburetors using a single fuel system in which a nozzle orifice is opened in a variable venturi tube of a slide throttle valve type disclosed in Utility Model Application No. 49-17682.
The advantage of using a single fuel system is that a fuel flow rate smoothly transitions from a low-speed operation range to an intermediate or high-speed operation range. Furthermore, the advantage of adding a mechanism for mechanically adjusting the fuel flow rate in response to the open-close operation of a throttle valve to such a system is that the air/fuel ratio is maintained within a preset range corresponding to the fuel flow rate and the air flow rate. Moreover, the introduction of bleed air is advantageous because it optimizes the fuel flow rate and improves formation of fine droplets of fuel discharged into the intake channel.
A mechanism for adjusting the fuel flow rate includes inserting a metering needle into a fuel nozzle adjusting the effective surface area and also represents the conventional technology. Moreover, in such a structure, bleed air is introduced between the main jet of a fuel passage and a nozzle orifice, and the flow rate of bleed air introduced into the fuel passage is determined by the difference in pressure between the bleed air inlet opening and nozzle orifice.
However, when the intake negative pressure generated during idling of general purpose engines was continuously measured, it was found that the intake negative pressure was not constant and was changing cyclically. Negative pressure acting in the nozzle orifice changes under the effect of these changes in the intake negative pressure. As a result, the difference in pressure between the nozzle orifice and bleed air inlet opening and the difference in pressure between the nozzle orifice and constant-fuel chamber also change, disturbing the air/fuel ratio in the air-fuel mixture supplied to the engine and, thus, destabilizing idling. Destabilization of idling causes cyclic degradation because it increases variations of the intake negative pressure and further destabilizes idling.
In engines for general applications, the quantity of discharge gases is small and the required fuel flow rate is low. Therefore, the effect produced by changes in the fuel flow rate during idling cannot be ignored.
The fuel system of the present invention was developed in particular to resolve the above-described problem of engine destabilization caused by changes in the intake negative pressure occurring during idling. It is an object of the present invention to equip a carburetor with a fuel system providing stable operation of the engine by constantly supplying thereto an air-fuel mixture with an air/fuel ratio within a preset range.
In order to resolve the above-described problems, a fuel system of the present invention comprises a single fuel passage leading from a constant-fuel chamber to a nozzle orifice opened into an intake channel, wherein a fuel adjusting part and a mixing chamber are provided in the fuel passage. The fuel adjusting part adjusts the effective surface area for passing the fuel with a metering needle executing linear reciprocal movement in response to the open-close operation of a throttle valve. Bleed air and the fuel that passed through the fuel adjusting part are introduced into the mixing chamber, which has a volume sufficient to absorb and cause a relaxation of changes of the negative pressure acting in the nozzle orifice. A mixture of the fuel and bleed air produced in the mixing chamber discharges from the nozzle orifice into the intake channel.
Controlling the fuel flow rate in a single fuel system by using a metering needle moving in response to the open-close operation of a throttle valve makes it possible to smoothly change the fuel flow rate over the entire operation range of the engine and to maintain the air/fuel ratio within the preset range by establishing correspondence with the flow rate of the engine intake air. Furthermore, since the flow rate of bleed air and fuel is determined by the difference in pressure between the bleed air inlet opening and the constant-fuel chamber or mixing chamber, the bleed air and fuel are suction introduced into the mixing chamber by the stabilized negative pressure, which is practically unaffected by the variations of the intake negative pressure, and the air/fuel ratio is maintained in even more appropriate preset range, thereby providing for stable operation of the engine.
Further, objects and advantages of the invention will become apparent from the following detailed description and accompanying drawings.